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a massive star in the latter stages of stellar evolution that suddenly contracts and then explodes, increasing its energy output as much as a billionfold. Supernovas are the principal distributors of heavy elements throughout the universe; all elements heavier than
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star that varies, either periodically or irregularly, in the intensity of the light it emits. Other physical changes are usually correlated with the fluctuations in brightness, such as pulsations in size, ejection of matter, and changes in spectral type, color, or
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nova(noh -vă) (classical nova) A close binary star system in which there is a sudden and unpredictable increase in brightness by maybe 10 magnitudes or more (see table). Novae are a class of cataclysmic variable. In a typical spiral galaxy like our own, there are maybe 25 nova eruptions per year. The brightness increases to a maximum within days or sometimes weeks and then declines to a value probably close to its faint pre-nova magnitude, indicating that the eruption did not disrupt the bulk of the star. Fast novae usually increase in brightness by a factor of 105 in a few days, remaining at peak brightness for less than a week; they then decline steadily, initially quite rapidly, over several months. Slow novae reach maximum brightness more slowly and erratically, the increase being less than in fast novae, and then decline much more slowly. The total energy released, however, is about the same in both cases. Hydrogen-rich gas is ejected from the star resulting in the tremendous outflow of heat and light. The ejected matter forms a rapidly expanding shell of gas that can become visible as the nova fades. At later stages of the eruption the spectra of most novae show the bright forbidden lines characteristic of very low density emission nebulae.
Like other cataclysmic variables, a classical nova is a close binary system in which one component is a white dwarf. The other is a main-sequence star that is expanding to fill its Roche lobe (see equipotential surfaces) and is hence losing mass to the white dwarf: some of its gases ‘overflow’ to form a disk surrounding the white dwarf. The rate of overflow is about 10–9 solar masses per year, about ten times higher than in a dwarf nova system, and as a result the disk is always as bright as a dwarf nova at maximum. Before a nova explosion the hot turbulent disk is the brightest part of the system, and its light makes a pre-nova appear as a bluish irregularly fluctuating ‘star’. The hydrogen in the disk spirals down on to the surface of the white dwarf, and after a period of some 10 000 to 100 000 years enough has accumulated to react in a thermonuclear explosion – the nova outburst. The explosion leaves the system fundamentally unchanged, however, and the flow of gas resumes to reestablish the accretion disk around the white dwarf.
Novae are now named initially by constellation and year of observation; before 1925 they were numbered in order of observation. They are later given a variable star designation, as for example with DQ Herculis, i.e. Herculis 1934. See also recurrent nova; X-ray transients.
In many ways, Nova signaled the end of an era. The comic was an avowed attempt to recapture the innocence and excitement of the early days of the all-conquering 1960s Spider-Man comic. In the 1970s, the tide of comics was turning, however, and Spider-Man was swept away in the wave of X-Men mania; although he was fondly enough remembered to make several spirited comebacks. Nova was dreamed up in the 1960s by would-be comics pro Marv Wolfman, who was finally able to make his dream a reality when he became editor-in-chief of Marvel Comics. The Nova comic’s first issue, in 1976, introduced readers to underachieving high-school zero Richard Rider, who stumbles across a dying alien (shades of Green Lantern’s origin) by the name of Rhomann Dey, from the planet Xandar, who presents him with a golden helmet. In the time-honored tradition, the helmet gives the startled teen fantastic abilities, including amazing strength and the power of flight. In the guise of Nova the Human Rocket he can do the impossible.
The comic’s high school milieu would have been familiar to any longtime Spider-Man fan, but at least Rider had a reasonably functional family—as much a rarity in comics then as now. With solid artwork from veterans such as Sal Buscema and Carmine Infantino, the title was always attractive, and a bizarre mix of villains usually guaranteed the reader a good time. In addition to arch-foe the Sphinx, Nova also tussled with bargain-basement bozos such as Diamondhead, the Condor, Powerhouse, Mega Man, and the Corruptor. Nova’s twenty-five-issue run ended with our hero, along with fellow crime-fighters the Comet and Crime Buster (Team Nova, anyone?), in pitched battle with the Sphinx and assorted hoods, en route to Xandar.
The storyline was concluded in a few issues of The Fantastic Four (issues #208-#212), in which Nova and pals defended Xandar from a Skrull invasion, cosmic anti-hero Galactus battled it out with the Sphinx, and Nova was awarded the title of Xandar’s Protector. After a while in space, Rider felt homesick and was allowed to return, but at the expense of his powers—or so he (and readers) thought. After a decade in the wilderness, forgotten by his readers and now a depressed high school dropout, he resurfaced in the New Warriors, in 1990, courtesy of the team’s co-creator Tom DeFalco and series writer Fabian Nicieza. Nova rediscovered his powers in the most dramatic way possible—by being dropped from a tall building (by team leader Night Thrasher) and finding, as he hurtled to the ground, that his former powers kicked in, much to everyone’s relief. The New Warriors team, made up of sidekicks and rejects, proved to be one of the hits of the 1990s, renewing interest in Nova and inspiring a second stab at solo success in 1994.
Through a mistaken newspaper report in the team’s early days, Nova became widely known as Kid Nova, much to his chagrin and his teammates’ merriment—an incident typical of the New Warriors’ humorous undercurrent. Sadly, after the exuberance of the comic’s first few years, darker elements began to encroach on the fun and, in the last issues of his own (second) comic (issue #18 in 1995), Nova lost his powers (again). He had failed to answer a call to arms from the long-forgotten planet Xandar, and so an evil character, called Nova Omega, stripped the hero’s abilities. Rider eventually recovered his powers two years later, in the last issue of New Warriors, only to endure another period in the comics wilderness before Image founder Erik Larsen gave him one last try.
The Larsen-scripted Nova, The Human Rocket (1999), began with the New Warriors disbanding and then set about returning Nova to his roots, complete with revived villains the Sphinx, Diamondhead, and others. Now, more than twenty years after his first appearance, Rider had finally become a college student, and much as before, the comic was a mélange of misfit angst, rollicking adventure, and a zeitgeist-defying optimism. It lasted seven issues.
In the twenty-first century Nova was not present when some of his New Warriors comrades were killed in an explosion, and he later briefly joined the Secret Avengers. A new Nova series started in 2007, written by the team of Dan Abnett and Andy Lanning, who also wrote the 2010 limited series The Thanos Imperative, in which Nova plays a major role. Nova apparently sacrifices his life to defeat Thanos in the latter series, but such deaths in the Marvel universe rarely prove to be real or permanent. —DAR & PS
a star whose luminosity abruptly increases thousands or even millions of times (104 times on the average) and then slowly decreases. The greatest luminosity is observed for periods ranging from one to two hours (fast novae) to several days (slow novae). It takes several years for the luminosity to decline to the initial value.
The term “nova” originated in antiquity, when stars that became visible in the sky as a result of increase in brightness were thought to have been newly formed stars. Photographic studies have refuted this view: by the early 20th century it was proved that these stars exist before their outburst but have a considerably lower luminosity, to which they return after the outburst. The light curves of various novae resemble one another (Figure 1). During the period of maximum brightness, some novae have a magnitude of 1 or 2, sometimes higher. Such novae were observed in 1901 in the constellation Perseus, in 1918 in the constellation Aquila, in 1925 in the constellation Pictor, in 1934 in the constellation Hercules, and in 1942 in the constellation Pup-pis. As of the 1970’s, a total of more than 180 nova outbursts have been observed in the Milky Way Galaxy. According to statistical calculations, there are about 100 nova outbursts in the Galaxy each year, but only one or two are observable from the earth. Novae have also occurred in neighboring galaxies: 230 in the Andromeda Galaxy and 15 in the Magellanic Clouds.
The rise to maximum brightness is rapid, and consequently the light curve at this stage has been poorly studied. It is known that when the brightness reaches a value two magnitudes below maximum, the increase in brightness temporarily stops for several hours to several days.
The light curves of novae exhibit the greatest diversity in the transition stage, where three basic types of brightness variation are noted: (1) a gradual decrease in brightness, (2) large periodic oscillations, and (3) a deep minimum of several weeks’ duration followed by a partial restoration of brightness.
The changes in brightness are accompanied by large shifts in the spectra. Before their outburst, novae are hot stars of spectral class O or B. However, there are few observations of the spectra of novae before flareup.
As a nova approaches maximum brightness, its spectrum acquires features characteristic of high-luminosity stars of spectral class A or F, with narrow absorption lines displaced toward the short-wavelength end. This indicates that the outer layers of the nova’s atmosphere are expanding at a rate of about 1,000 km/sec; for slow novae, the rate is somewhat less. Immediately after maximum, emission lines, mainly of hydrogen and ionized metals, appear in the spectrum. The decline in brightness is accompanied by intensification of the emission lines, as well as the appearance of new absorption lines. This is associated with an additional ejection of material. When the star’s brightness decreases five magnitudes, the nova’s nebular stage begins, and its spectrum at this time strongly resembles that of a planetary nebula. The nebular stage lasts for several years. Many years after the outbursts, the spectra of novae resemble those of white dwarfs.
Nova outbursts are connected with a loss of stability in a star’s outer layers and the ejection of material. However, the explosions do not affect the star as a whole. The fraction of the star’s mass ejected during the outburst averages about 10–5 of the star’s mass, or about 1028g. The total energy of a nova outburst is equal to about 1045 ergs, or 1038 joules. The star’s envelope is ejected either at the very start of the outburst, that is, when the brightness begins rising, or, according to the Soviet astronomer E. R. Mustel’, at maximum brightness. In the latter case, the increase in brightness is related to the expansion of the star itself, which begins to contract after the maximum. The appearance of bright emission lines and other features in a nova’s spectrum after maximum is caused by processes originating in the ejected envelope. The spectral emission lines arise as a result of both the envelope’s absorption of light from the very hot exposed layers of the star and the interaction of atoms in the envelope with high-speed particles emitted by the star for some time after maximum brightness. As the envelope expands, its density decreases and it becomes more ionized. At a density of about 10-19 g/cm3, spectral lines characteristic of a highly rarefied gas appear, indicating the onset of the nebular stage.
Several years after the outburst, the envelopes ejected by many novae expand great distances from the stars and become visible from the earth. As a rule, they are inhomogeneous and form two large clumps, called polar condensations, in opposite directions from the star. The star’s magnetic field may play an important role in the formation of the envelope’s shape: if this field, as is proposed, has a dipole character, then the ejection of material occurs primarily along the axis connecting the star’s magnetic poles. We can determine the distance to a nova by means of data on the angular velocity of expansion of nova envelopes and the velocity of expansion obtained from an analysis of the envelope’s spectrum.
It was discovered in the 1950’s that novae are members of close binary-star systems, in which the distance between the components is of the order of the radii of the stars themselves. The second components of the pairs are cooler stars. The study of binary systems with novae has made possible the first reliable estimates of the masses of novae. It turns out that, on the average, nova masses do not differ significantly from the mass of the sun.
The luminosities of novae in the Galaxy cannot be determined with great accuracy. One of the principal methods of estimating the luminosities at maximum brightness is provided by an empirical relation between the absolute stellar magnitude at maximum and the rate of decrease after maximum: the higher the maximum, the faster the brightness diminishes; novae are classified as fast or slow on the basis of the rate of decrease in brightness. This relation has the form
Mv, max = –11.5 + 2.5 log t3
where Mv, max is the nova’s absolute visual magnitude at maximum and t3 is the time (in days) it takes for the star’s brightness to decrease three magnitudes. This relation is satisfied not only by novae in our Galaxy but also by those in the Andromeda Galaxy and the Magellanic Clouds. The average absolute visual magnitude of novae at maximum is
Mv = –7.3 magnitude
Thus, novae are the brightest objects in our Galaxy, after super-novae. Because of their high luminosity, novae serve as indicators of distances to nearby galaxies. At minimum brightness, the absolute magnitude of a nova is relatively faint; on the average Mv, min = +3.5 magnitude. In some stars, the light at minimum is determined by the cooler component, which at this stage is brighter than the nova itself. By all parameters—mass, luminosity, and size—novae in the stable state are dwarfs.
Recurrent novae do not significantly differ from regular novae, except for the speed with which the star returns to its prenova state. This time is usually about one year. As of the 1970’s, 11 recurrent novae are known. Of these, the star T Pyxi-dis experienced the greatest number of outbursts (five) in the period 1890 to 1967.
In the late 1960’s it was discovered that novae emit intense infrared radiation, which increases in intensity as the brightness diminishes. In the novae observed during this time, maximum infrared radiation was recorded about 100 days after maximum brightness in the visible region of the spectrum. It is possible that this infrared radiation is caused by heated dust particles ejected by the nova or formed in the ejected envelope.
The causes of nova outbursts are still not very clear. However, there is no doubt that the outbursts are the result of increased instability within dwarfs of small mass. Most contemporary hypotheses view a nova outburst as a thermal explosion occurring as a result of a disruption of the thermal equilibrium in the deep inner layers. The shock wave from the explosion travels to the star’s surface with velocities of the order of 1,000 km/sec and tears off the outer layers of the photosphere. Similar hypotheses were developed by the Soviet astronomers A. I. Lebedinskii and L. E. Gurevich, the French astronomer E. Schatzmann, and others. According to Schatzmann, the outburst is caused by the accumulation in the star’s interior of the isotope 3He, which leads to a nuclear explosion within the star; the isotope is destroyed during the explosion but then accumulates anew, which may explain the recurrence of the outbursts. After novae were discovered to be binaries, hypotheses were advanced linking outbursts to the structure of close binary stars. According to the hypothesis proposed by Schatzmann (1958), the coincidence of the orbital period with the period of natural oscillation of one of the binary components may lead to an explosion with an ejection of material both in the direction of the perturbing companion and in the opposite direction; this is how the observed shapes of nova envelopes are explained.
The place of novae in the general scheme of stellar evolution has not been established with certainty. However, there is no doubt that nova outbursts occur in the late evolutionary stages of stars, probably binaries. The outbursts may precede a star’s transformation into a white dwarf.
REFERENCESVorontsov-Vel’iaminov, B. A. Gazovye tumannosti i novye zvezdy. Moscow-Leningrad, 1948.
Zvezdnye atmosfery. Edited by J. Greenstein. Moscow, 1963. Chapter 17. (Translated from English.)
Eruptivnye zvezdy. Moscow, 1970. Chapter 1.
Payne-Gaposchkin, C. The Galactic Novae. Amsterdam, 1957.
V. P. ARKHIPOVA
Memory could be accessed indirectly through addresses stored in other memory locations. If locations 0 to 3 were used for this purpose, they were auto-incremented after being used. If locations 4 to 7 were used, they were auto-decremented. Memory could be addressed in 16-bit words up to a maximum of 32K words (64K bytes). The instruction cycle time was 500 nanoseconds(?). The Nova originally used core memory, then later dynamic RAM.
Like the PDP-8, the Data General Nova was also copied, not just in one, but two implementations - the Data General MN601 and Fairchild 9440. Luckily, the NOVA was a more mature design than the PDP-8.
Another CPU, the PACE, was based on the NOVA design, but featured 16-bit addresses (instead of the Nova's 15), more addressing modes, and a 10-level stack (like the Intel 8008).